The present invention relates generally to fiber optic cables and, more particularly, to fiber optic cables, such as indoor/outdoor cables, that have both water blocking and flame retardant properties.
Fiber optic cable generally must have different properties and meet different requirements depending upon the intended application of the cable. For example, fiber optic cable that is intended for outdoor use generally includes water blocking elements to prevent the infiltration of water and, in instances in which water does penetrate the cable, to mitigate the effects of the water. In contrast, fiber optic cables that are intended for indoor use do not generally have to be designed to be water blocking, but are commonly required to be flame retardant. Moreover, some fiber optic cables are intended for indoor/outdoor use and, as such, must have both water blocking and flame retardant properties.
A fiber optic cable generally includes one or more buffer tubes, one or more optical fibers disposed within each buffer tube and an outer jacket surrounding the buffer tube(s). Fiber optic cables that are intended for outdoor or indoor/outdoor use must also include elements to prevent or mitigate the effects of water that may infiltrate either the space between the outer jacket and the buffer tube(s) or the interior of the buffer tube(s). In this regard, fiber optic cables that are intended for outdoor or indoor/outdoor use include water blocking elements to prevent the longitudinal migration of water through the fiber optic cable such that any water that might infiltrate the cable does not reach the cable closures, connectors or other electro-optical equipment to which the fiber optic cable is connected and which may be damaged by exposure to water.
In order to prevent water migration through the interior of a fiber optic cable, a fiber optic cable typically includes a grease-like filling compound that fills the interior passageways defined by the cable. In particular, the grease-like filling compound typically fills the buffer tube(s) as well as any space between the buffer tube(s) and the outer jacket. However, use of a grease-like filling compound presents a number of disadvantages. For example, the grease-like filling compound is relatively messy and complicates splicing or other connection of the optical fibers of the fiber optic cable. In this regard, the grease-like filling compound must be removed from the end portions of the optical fibers and the optical fibers must be cleaned prior to installation, splicing or other connection of the optical fibers. As will be apparent, cleaning of the grease-like filling compound from the optical fibers is a time-consuming process requiring chemical solvents which, in turn, creates additional problems relating to their subsequent disposal.
In an attempt to overcome at least some of the disadvantages associated with the use of grease-like filling compounds, dry fiber optic cables have been developed that do not include grease-like filling compounds and, instead, include other types of water blocking elements including water swellable yarns and tapes. These water swellable elements generally include water swellable particles that swell upon exposure to water so as to form a blockage in the cable that prevents the further longitudinal migration of the water through the cable. Typically, the water swellable particles are formed of a superabsorbent polymer. As known to those skilled in the art, superabsorbent polymers are partially cross-linked polymers that absorb many times their own weight in water and swell considerably without dissolving. Examples of superabsorbent polymers are acrylate, urethane or cellulosic-based superabsorbent materials. By way of example, a water swellable tape often includes a plurality of water swellable particles sandwiched between a pair of nonwoven fabric layers. Alternatively, a water swellable yarn can be coated or impregnated with water swellable particles.
In order to prevent water migration through a buffer tube, optical fibers within the buffer tube can be wrapped with a water swellable tape or yarn. Likewise, to prevent water migration between the outer jacket and the buffer tube(s), the buffer tube(s) can be wrapped with a water swellable tape or yarn. While effective in preventing water migration through a cable, the water swellable tapes or yarns that are wrapped about the buffer tube(s) in order to prevent water migration between the outer jacket and the buffer tube(s) may disadvantageously increase the size of the fiber optic cable.
While water exposure and the prevention of water migration is not a substantial concern, indoor cables must be designed to be flame retardant. As such, indoor cables, as well as indoor/outdoor cables, can include a tape formed of a flame retardant material, such as mica, that is wrapped about the buffer tube(s) prior to extruding the outer jacket thereover. Moreover, indoor cables can include an outer jacket formed of a flame retardant plastic material.
Due to their use in both interior and exterior applications, indoor/outdoor cables must generally include both water blocking and flame retardant properties. Within a buffer tube, an indoor/outdoor cable generally includes a grease-like filling compound in order to prevent water migration therethrough. Moreover, wrapped about the buffer tube(s) in the space between the buffer tube(s) and the outer jacket, an indoor/outdoor cable generally includes a tape formed of a flame retardant material, such as mica, sandwiched between a pair of water swellable tapes. The flame retardant tape will provide the necessary flame retardant properties, while the water swellable tapes prevent water migration along the length of the fiber optic cable in the space between the outer jacket and the buffer tube(s). Unfortunately, the grease-like filling compound complicates installation and splicing of the fiber optic cable, while the water swellable tapes disadvantageously increase the overall size of the fiber optic cable. Moreover, the cable elements required to provide both water blocking and flame retardant properties for an indoor/outdoor cable increase the number of cable components which, in turn, increases the complexity of the fabrication process as well as the resulting cost of the fiber optic cable. Thus, it would be advantageous to provide a fiber optic cable having both water blocking and flame retardant properties, such as for use in indoor/outdoor applications, which was craft-friendly and which reduced the number of cable components which, in turn, may reduce the size and cost of the fiber optic cable.
An improved fiber optic cable having both water blocking and flame retardant properties is therefore provided. As such, the fiber optic cable is particularly useful for indoor/outdoor applications. According to one aspect of the invention, the fiber optic cable includes a buffer tube, at least one optical fiber disposed within the buffer tube, a composite tape surrounding the buffer tube that comprises a layer formed of a flame retardant material and a first coating that is formed of a water swellable material and that is disposed on one surface of the flame retardant layer, and a jacket surrounding the composite tape. In the preferred embodiment, the composite tape further comprises a second coating formed of a water swellable material that is disposed on the opposite surface of the flame retardant layer from the first coating. Advantageously, the water swellable coatings of the composite tape prevent water migration between the jacket and the buffer tube while the flame retardant layer of the composite tape provides fire resistance.
The fiber optic cable can also include a water blocking element disposed within the buffer tube for preventing water migration through the buffer tube. In one embodiment, for example, the fiber optic cable includes a filling compound disposed within the buffer tube and surrounding the at least one optical fiber. Alternatively, the fiber optic cable can include a tape comprised of a water swellable material that at least partially surrounds the at least one optical fiber.
According to another aspect of the present invention, a dry fiber optic cable is provided that includes a buffer tube comprised of a flame retardant material, at least one optical fiber disposed within the buffer tube, a first tape comprised of a water swellable material that at least partially surrounds the at least one optical fiber, at least one roving extending lengthwise along the buffer tube and having a coating comprised of a water swellable material, and a jacket surrounding the buffer tube and the at least one roving. As such, the buffer tube provides flame retardant properties. In addition, the first water swellable tape prevents water migration through the buffer tube, while the water swellable coating of the at least one roving prevents water migration between the jacket and the buffer tube. According to either aspect of the present invention, the fiber optic cable advantageously provides both flame retardant and water blocking properties and is therefore especially well suited for indoor/outdoor applications.